The present invention relates to a DC magnetic-based apparatus and means to detect—even in the presence of conductive and ferrous metals as well as stray electromagnetic fields—the three-dimensional location of medical instruments within the human body and use this information for guidance purposes in image-guided procedures.
It provides an optimal means of performing electromagnetic guidance within image-guided medical procedures. Based on the principles of pulsed DC magnetic tracking, a panoply of sensors and transmitters is provided to address the requirements of myriad interventional and diagnostic procedures. Procedural requirements commonly include tracking above a metal bed, tracking a catheter inside a patient, and even hybrid tracking methods for localizing miniaturized sensors over the full length of a human body. Once optimal components are selected, system parameters are optimized. These parameters, which directly affect the accuracy, resolution, dynamic performance, and stability of the system, are determined at the time of the procedure. This ensures that environmental factors, including magnetic-field distortion by conductive and ferrous materials and electromagnetic interference, are minimized before the start of the procedure. Optimizations can be performed either manually or automatically.
Minimally invasive, image-guided medical procedures are becoming increasingly commonplace because they reduce patient trauma and costs by condensing both the size of incisions and operating times, they yield better clinical outcomes and reduced hospital stays. The most widespread example is laparoscopic cholecystectomy in which narrow tube-like instruments, holding miniaturized cameras and surgical tools, are inserted through keyhole openings in the abdomen for fast and efficient removal of a diseased gallbladder. In a growing number of minimally invasive applications within organs and vascular structures, however, miniaturized cameras and tools are often insufficient to accomplish the procedure. In these cases, the physician cannot always see where his instrument is located or its direction to a known landmark. Often he must rely on one or more two-dimensional imaging modalities, such as X-rays, fluoroscopy, computed tomography (CT), magnetic resonance imaging (MRI) or ultrasonography. These scans are not aligned to the coordinate frame of the patient and must be mentally stacked together to appreciate the three dimensionality of the patient's anatomy. As a result, guiding instruments to internal targets can become awkward and difficult to achieve when relying on imaging alone. Computer-assisted techniques are often applied to scan planes and render 3D reconstruction of image planes, but they do not solve the 3D guidance problem. The physician is still confronted with the problem of determining where his instrument and his medical target are located in image space. Currently, the most common imaging modality used for instrument guidance is fluoroscopy. While it provides real-time imaging, the results are in two-dimensions only. In addition to limiting the physician's three-dimensional perspective, it further exposes him and patient alike to the health risk of ionizing radiation.
Mechanical, ultrasonic, optical, magnetic resonance, X-Ray, first-generation AC magnetic and second-generation DC tracking technologies have been applied to the image-guidance problem with limited success.
Applicants are aware of the following tracking modalities in the prior art that have been applied to the problem of medical visualization and guidance.
U.S. Pat. No. 4,794,931 to Yock [Cardiovascular Imaging Systems, Inc.: “Catheter Apparatus, System and Method for Intravascular Two-Dimensional Ultrasonography”] discloses an ultrasonic method of achieving high-resolution intravascular imaging, preferably for performing atherectomies. The invention provided an early means of guiding a medical instrument into branches of blood vessels by embedding an ultrasonic crystal in the distal tip of a 9 French catheter, which radiates energy off a reflector into tissue immediately opposite the tip cutout. The resulting ultrasonic image provided a two-dimensional means of visualizing the cross section of a vessel wall for assessing plaque build up and degree of stenosis.
U.S. Pat. No. 5,899,860 to Pfeiffer [Siemens: “Method and Device for Determining the Position of a Catheter Inside the Body of a Patient”] discloses a positional system for catheters. It cannot localize multiple instruments or complimentary instruments such as flexible endoscopes, probes, and long needles. The disclosure broadly identifies a number of energy transmission and reception schemes for localization. Among these are: piezo elements for ultrasonic machines; electromagnetic coils; and Hall-effect generators. The approach is conceptual and does not address real world issues of interference and distortion or the need to find a full six degrees-of-freedom (position and orientation) solution to the localization problem.
U.S. Pat. No. 6,442,417, Shahidi, [“Method and Apparatus for Transforming View Orientation in Image-Guided Surgery”] describes a system and method for increasing the range of motion of an instrument, such as an endoscope, once it is inserted within the patient. The invention enables a physician to increase view orientation for improved observation of an internal target. The patent, does not address the tracking of instruments once inside the body, stating merely that robotic, mechanical, acoustic, optical or magnetic approaches may be applied to navigate an instrument to an internal site.
Mechanical tracking approaches have been applied to the guidance problem. These provide exceptional accuracy but are hampered by cumbersome mechanical linkages that interfere with physician motions and instrument maneuverability. They also have difficulty tracking multiples points and handling sterilization requirements. Their greatest problem is inability to track flexible instruments inside the patient.
U.S. Pat. Nos. 5,383,454 and 5,851,183 to Bucholz, [“System for Indicating the Position of Surgical Probe Within a Head or an Image of the Head”] generally disclose mechanical and optical devices for navigating a surgical probe—in neurosurgery. They are based on a stereotactic frame (U.S. Pat. No. 5,383,454) and an optical scanning technique (U.S. Pat. No. 5,851,183) employed to aim a surgical probe at targets inside the brain, referenced to pre-established coordinate points. Another mechanical approach is an automatic apparatus for computer-controlled stereotactic brain surgery as described in U.S. Pat. No. 5,078,140 to Kwoh. Its needle guide is integrally connected to the stereotactic apparatus, thereby allowing the physician to choose the most suitable trajectory of the needle toward the target. This is a highly-complicated expensive system, requiring recalibration for each procedure.
Optical devices constrain the physician to maintain a clear path (unrestricted line-of sight) between a source of radiated energy (e.g., light or infrared energy emitted from active markers or light or infrared energy reflected from passive markers) and optically sensitive detection arrays, such as charge-coupled devices (CCD). In a busy and crowded operating room, maintaining a clear path between emitting and detecting elements is not always practically possible. Because the emitter or detector is placed on the proximal end of a rigid instrument, the system must calculate an offset for accurate distal tip measurements. This allows errors to creep into the measurements and cannot account for bending of the instrument during a procedure. Also, the lever effect magnifies small errors at the proximal end into large, sometimes unacceptable errors at the distal tip. For flexible instruments (such as catheters and endoscopes) fully inserted within the body, optical tracking devices are impractical.
Optical patents, such as U.S. Pat. No. 5,617,857 to Chader [Stryker: “Imaging System Having Interactive Medical Instruments and Methods”] generally disclose an imaging system in which a medical instrument is tracked by optical means. In this system, light emitting diodes (LEDs) are attached to the instrument referenced to a nearby bank of detectors. The system is connected to a computer display so that the location of the instrument relative to a pre-acquired image of the patient's anatomy can be viewed. Again because a clear line of sight must be maintained between emitters and detectors, it cannot be used to track flexible scopes and catheters inserted inside the body.
U.S. Pat. No. 6,167,296 to Shahidi [“Method for Volumetric Image Navigation”] discloses a computer-driven navigation system connected to a surgical instrument for the purpose of locating instruments in real time and displaying such information on a computer display. It specifies an optical position tracking system employing LEDs and detectors to provide real-time instrument location and means to register images with respect to the patient, and imaging software for reconstruction of 3D images of pre-acquired scans. While an optical system is the preferred embodiment, the inventor states that a sonic tracking system can also be employed. Both approaches require the aforementioned clear line of sight between sources of energy (light or acoustic) and detectors mounted on instruments.
A magnetic resonance imaging (MRI) system is a complex, expensive imaging modality whose signals have been applied to monitoring the position of a specially configured catheter within the body. It has become an attractive approach for research purposes because it offers superior soft tissue contrast and excellent capability for functional testing. Due to the expense, complexity and health issues (i.e., intravascular heating) related to this approach, it has not been used for generalized 3D localization in image-guided procedures.
U.S. Pat. No. 5,318,025 to Domoulin [GE Medical, “Tracking System to Monitor the Position and Orientation of a Device Using Multiplexed Magnetic Resonance Detection”] anticipated the need for 3D instrument localization and developed a catheter containing receiver coils sensitive to magnetic resonance signals. Since detected signals are substantially proportional to the location of the coil along the line of the MRI field gradient, they are used to determine the catheter's position and orientation within the body. This localization procedure requires scheduling time in the MRI suite and cost per procedure is prohibitive for general-purpose image guidance. Other drawbacks include the requirement to inject contrast agents, the need to remove metallic equipment from the MRI suite, and the inability of the system to image from off-axis angles for optimal viewing.
Real time X-Ray technology, i.e., fluoroscopy, is the de facto standard for localization and guidance of instruments within the body. Serious restrictions, as stated above, include: two-dimensional imaging, reliance on use of contrast agents, expense and radiation exposure. Fluoroscopic proponents and critics alike have unanimously called for less reliance on this modality. Once a viable 3D guidance device is fielded for image-guided procedures, fluoroscopy for localization purposes will most likely be limited to calibration and verification of information provided by non-ionizing means.
In addition to these approaches, both alternating current (AC) and direct current (DC) magnetic field generating and sensing technologies have been applied to the medical guidance problem.
For the purposes of categorization, AC magnetic technology and its many derivative implementations are defined as first-generation magnetic tracking. The technology first gained the interest of medical researchers because of its capability to track sensors without line-of sight restrictions, thus enabling trackable sensors to be inserted into the body without occlusion or data loss. Operationally, these systems read induced sensor voltages referenced to one or more magnetic fields and measure near-field magnetic field vectors. Typically one or more magnetic coils in a sensing assembly provide sufficient information to solve its position and two or three angular rotations relative to a dipole transmitter whose two or three coils are sequentially or simultaneously energized. Non-dipole transmitters can also be employed.
First-generation AC electromagnetic tracking is based on transmission and sensing of AC magnetic fields first patented in 1975 (Kuipers, U.S. Pat. Nos. 3,868,565 and 3,983,474) and 1977 (Raab, U.S. Pat. No. 4,054,881). While solving the aforementioned line-of-sight problem of acoustic and optical systems, the technology is acutely sensitive to measurement distortion from common hospital metals, such as electrically conductive metals (e.g., 300-series stainless steel, copper, titanium, aluminum and carbon composites) as well as ferrous metals (e.g., iron, steel and certain nickel alloys). In the presence of these metals, AC field waveforms, which are constantly changing, produce circulatory (eddy) currents in nearby metals that generate secondary fields distorting field patterns. These spurious fields spawn additional sources of magnetic fields resulting in measurement errors in the sensor. To address the restriction of tracking in regions free of conductive and ferromagnetic metals, a number of approaches have been patented to deal with the problem. Among these are: application of mapping and compensation techniques (Raab et al.), implementation of mathematically derived correction factors to measurements (Anderson, U.S. Pat. No. 6,774,624), compensation by measuring and adjusting phase shifts detection metal (Acker et al), shielding of distorters (Anderson—U.S. Pat. No. 6,636,757; Jascob, U.S. Pat. No. 6,636,757), signal processing of eddy current effects (Seiler, U.S. Pat. No. 6,836,745), sounding of warning signals when a distorter is detected (Kirsh, U.S. Pat. No. 6,553,326) etc. Despite the development of these and other AC distortion control strategies, AC systems still require that a physician adopt a number of workarounds and, in some cases, procedural changes to handle the metal problem. These include, among other things, requiring the physicians to use expensive non-metallic instruments and non-metallic operating tables, performing procedures within the confines of large sets of obtrusive coils, engaging in costly and tedious set-up/calibration procedures, and restricting the range and motion of physicians, instruments and equipment.
Representative AC magnetic patents applied to medical imaging include: U.S. Pat. No. 6,233,476 to Strommer et al. [Mediguide: “Medical Positioning System”]. It discloses a medical device employing an AC magnetic sensor for determining the position and orientation of a surgical probe relative to a reference frame in association with an imaging system. In the preferred embodiment, it claims to overcome the disturbing effects of metal objects by employing a system in which a plurality of electromagnetic fields are generated and sensed. The implementation, however, is costly and subject to numerous transmitter signal pick-up errors by its sensors, which produce noise in outputs and limit its general use in image-guided procedures.
U.S. Pat. No. 6,690,963 [“System for Determining the Location and Orientation of an Invasive Medical Instrument”], issued to Ben-Haim of the Biosense Webster division of Johnson & Johnson is representative of many AC magnetic tracking variations and techniques to achieve 3D magnetic guidance of image-guided procedures.
U.S. Pat. No. 6,836,745 to Seiler [NDI “Method for Determining the Position of a Sensor Unit”] discloses a means of reducing metallic distortion from conductive metals in five degrees-of freedom AC magnetic tracking systems. It claims to correct these distortions by measuring the location of electrically conductive objects and entering this data into a computer program, which calculates the eddy currents and the resulting field distortions. These distortions are then defined in the coordinate system defined by the AC field transmitter and the interference field generated by the eddy currents is nulled. The patent claims that mathematical models are then used to form a correction to the error. In practice, a method that tries to calculate eddy currents as a “virtual source” will yield an overall improvement in reducing distortion but the error is unlikely to reach zero. Such a system also suffers from noise issues if it attempts to overly compensate for the conductive metal. The inventors acknowledge that the system cannot always totally eliminate conductive metal distortion and make no claims to correcting for ferromagnetic metals commonly found in operating rooms.
U.S. Pat. No. 6,636,757 [Medtronic: “Method and Apparatus for Electromagnetic Navigation of a Surgical Probe Near a Metal Object”] to Jascob claims a method and apparatus for AC electromagnetic navigation of a surgical probe near a metal object. It positions a shield near a metallic object in an attempt to reduce field distortion. The chief limitation to this approach is the ubiquity of metal in the operating room. Because the system generates and senses AC fields, multiple obtrusive shields must be instrumented for each procedure. Indeed Jascob acknowledges in his preferred embodiment that the system must shield multiple objects, such as: the operating room table, fluoroscope, microscope, high intensity focused ultrasound system, multiple ultrasound probes, intraoperative CT and MRI machines, surgical robotic equipment, and even metal trays. Further the system assumes that the shielding is placed on metallic objects that remain static or stationary. Once an objects moves, however, it must be recalibrated. This is an unrealistic requirement due to the constant movement of clinicians and equipment as well as contamination rules that demand that nothing be touched or moved in the sterile field around a patient.
U.S. Pat. No. 6,774,624 to Anderson et al. [GE Medical Systems: “Magnetic Tracking System”] offers a theoretical dissertation on a broad range of modeling and shielding techniques to moderate metallic distortion caused by eddy currents in AC electromagnetic systems. In one aspect of the invention, a conductive shield is disclosed, configured to fit about or contain an interfering component or piece of equipment. The shield standardizes the magnetic field disturbance introduced by the component.
To address the metal sensitivity problem of AC magnetic trackers, second-generation technology, employing pulsed DC magnetic field generation and sensing, was first patented in 1989 (U.S. Pat. No. 4,849,692 to Blood) and 1990 (U.S. Pat. No. 4,945,305 to Blood). It provides six degrees-of-freedom tracking while overcoming critical conductive metal distortion deficiencies of first-generation AC magnetic technology. Using a fluxgate, it takes advantage of the steady state of pulsed DC waveforms to measure the field at an instant in time when eddy currents are not being generated in nearby metals. Accurate measurements may therefore be made in medical environments rich in conductive metals. In particular, it is inherently insensitive to medical type metals such as 300-series stainless and titanium, even when operating at a high measurement rate. DC is also capable of driving other conductive metal errors to zero by appropriate measurement rate reduction. In most AC based systems, the eddy current error can only be reduced slightly with decreased operating frequency.
While second-generation DC technology functions well in many medical applications, such as in the 3D localization of ultrasound probes, it faces a number of issues—sensor size and cost, complexity and limited transmitter options—that reduce its effectiveness and applicability in image-guided procedures.
Patented medical applications employing second-generation DC technology include:
Additional U.S. Pat. Nos. 6,626,832, 6,216,029 and 6,604,404 to Paltieli [UltraGuide] were reduced to practice in the UltraGuide 1000 image-guided system. It employed second-generation DC magnetic tracking technology to correlate the location of an ultrasound scanhead tracked with an 8-mm DC sensor and a long needle tracked at its proximal with a second 8-mm DC sensor. The combination allowed the physician to select a point and angle for needle insertion into the patient's body for visually-aided targeting purposes. Because the system was based on second-generation magnetic technology, miniaturized sensors were not available for insertion in the tip of the long needle. Instead a sensor was mounted on the proximal end, thus requiring a calibration procedure to calculate the tip of the needle referenced to the center of the sensor at its far end. Additionally, the system lacked a reliable means of determining whether flexure of the needle occurred during the procedure, since any misalignment of the tip of the needle versus its sensor location results in mis-targeting. Paltieli's U.S. Pat. No. 6,626,832 was developed as a means of detecting the bending of the medical instrument once inserted into the human body. At the time the system was introduced, only a single DC transmitter (not designed for imaging applications) was available. This made it difficult for early UltraGuide implementations, which mounted the transmitter in a standoff chassis, to overcome ferrous metal distortion and achieve high accuracy performance.
The present invention addresses these and other critical tracking issues in the prior AC and DC magnetic tracking art that must be solved for 3D guidance to be easily implemented and accepted within the medical community.
It accomplishes this purpose, as explained below, by applying third-generation magnet field generation and sensing technology to the medical guidance problem. By specifically addressing metal and noise—as well as sensor size and cost issues—it offers a significant improvement over current methods of localizing instruments within the human body. Its integration with advanced imaging modalities and imaging software further allows three-dimensional localization data to overcome the inherent limitations of visualizing 3D anatomy with 2D imaging tools.
The application of third-generation magnetic technology will serve many medical purposes. Of primary interest is its capability of synchronizing instrument tip with internal anatomy; of providing 3D reference points superimposed on imaged parts; of mapping and locating anatomical features; of navigating tools to pre-identified locations; of providing instantaneous feedback; and of facilitating the delivery of therapies to targets deep inside the body. Exemplary but by no means inclusive procedures benefiting from third-generation tracking technology include: endograft localization for treatment of abdominal aortic aneurysms, guidance of ablation probes to deep-seated tumors, 3D localization of robotic end-effectors to avoid collisions, in vivo quantitative assessment of pathology and, mapping of locations for implantation of radioactive seeds in soft tissue, 3D guidance for improved visualization in colorectal cancer screening, as well as guidance of diagnostic and therapeutic catheters and probes, such as endoscopes, laparoscopes, colonoscopes and bronchoscopes, to organs and structures within the human body.